Created by: luotao1

Reason

• https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/fluid/layers/nn.py is very long, it has 17800+ lines, but only shows 12000+ lines on GITHUB. And It's slow to open this url.
• There is error to blame this file on GITHUB. If you click https://github.com/PaddlePaddle/Paddle/blame/develop/python/paddle/fluid/layers/nn.py

This PR

• Split some APIs from nn.py to rnn.py and sequence_lod.py.
• TO avoid conflicts, split some other APIs from nn.py to math.py and loss.py.